1. Field of the Invention
This invention relates to a gas refining system for removing hydrogen sulfide contained in high-temperature and high-pressure reducing gases such as gas produced in a coal gasification process. More particularly, it relates to a gas refining system wherein the adverse effect of the heat of regeneration reaction on the adsorbent used in the fixed-bed desulfurizers can be avoided positively and easily.
2. Description of the Related Art
In recent years, the diversification of fuels (or raw materials) is advocated because of the exhaustion and rising cost of petroleum resources, and the development of techniques for utilizing coal and heavy oils (e.g., tar sand oil, shale oil, Taching heavy oil, Maya crude oil or vacuum distillation residue) has been promoted. As an example, attention has been paid to techniques for gasifying coal and heavy oils to produce fuels for use in electric power generation or the like and raw materials for use in syntheses. However, such gases obtained by the gasification of coal and heavy oils contain several hundred to several thousand parts per million of hydrogen sulfide, which must be removed for the purpose of preventing environmental pollution or protecting downstream equipment (e.g., gas turbines) against corrosion. Among the processes for removing hydrogen sulfide is a dry gas refining process which is advantageous from the viewpoint of thermal economy and simple in construction. This gas refining process is such that an oxide of a metal (e.g., Fe) is used as an adsorbent, sulfur compounds contained in a gas are adsorbed and removed by the adsorbent within a desulfurizer in the form of a sulfide, and the adsorbent having reduced adsorption capacity is regenerated by roasting it with an oxygen-containing gas. As the desulfurizer packed with the adsorbent, there is used not only a desulfurizer of the so-called fluidized-bed or moving-bed type, but also a desulfurizer of the fixed-bed type which has various advantages such as quick responsiveness to load variations, great adaptability to varying type of coal, and the simple construction and small size of the apparatus.
As to the way of dealing with the gas containing sulfur dioxide (SO.sub.2) formed during regeneration, there is a process in which the sulfur dioxide is reduced by a gaseous reduction technique and recovered (or discharged from the system) in the form of elemental sulfur, as described, for example, in Japanese Patent Provisional Publication No. 2-237613. In this case, air (or an oxygen-containing gas) is supplied at only one site for the purpose of regeneration.
Moreover, as described, for example, in Japanese Patent Provisional Publication Nos. 63-123801 and 1-254226, there is a process in which the regeneration gas containing sulfur dioxide formed by the roasting reaction is introduced into a reactor where, it is brought into contact with a calcium compound-containing slurry to effect the absorption and oxidation of sulfur dioxide and the precipitation of gypsum within the reactor. Thus, gypsum is recovered (or discharged from the system) as a by-product. In this case, air (or an oxygen-containing gas) needs to be supplied at two sites for the purposes of regeneration and oxidation within the aforesaid reactor.
However, the above-described conventional gas refining techniques involve the following problems.
(1) First of all, as can be seen from the above-mentioned patent publications, no particular control over the temperature of the adsorbent is exercised. Consequently, when a fixed-bed desulfurizer is used, there is a possibility that the temperature of the adsorbent may exceed its allowable limit to produce undesirable effects such as deterioration or powdering thereof, resulting in a marked reduction in desulfurization performance. More specifically, in the case of an adsorbent comprising, for example, iron oxide (Fe.sub.2 O.sub.3), its temperature limit is in the range of 650 to 700.degree. C. even though it has been subject to a treatment for imparting thermal resistance thereto. If this temperature limit is exceeded, the adsorbent may be deteriorated to cause a sharp degradation in properties associated with adsorption performance (e.g., specific surface area, pore volume and particle diameter), or may be powdered. Then, not only the desulfurization performance is reduced, but also dust is newly produced in the desulfurizer to seriously interfere with the decontamination of gases obtained by the gasification of coal and the like. In the case of a fixed-bed desulfurizer, undesirable effects such as deterioration or powdering of the adsorbent during operation need to be avoided with high reliability because it is difficult to exchange the adsorbent.
(2) Moreover, especially in gas refining processes wherein gypsum is recovered from the regeneration gas, it is difficult to control the supply of air because air (or oxygen) is supplied at two sites. Accordingly, careful consideration must be given from the viewpoint of protection of the adsorbent. That is, there is a possibility that, since the temperature of the adsorbent during its regeneration varies according to the feed rate of air, an excessive supply of air may cause the temperature of the adsorbent to exceed the aforesaid allowable temperature range.